A recent environmental problem includes the issue concerning the fuel consumption efficiency of a vehicle, and, as environmental protection is becoming important, fuel consumption regulations for vehicles have been strengthened. As a scheme for improving fuel consumption efficiency, various methods for reducing the weight of a vehicle have been considered from various angles. A current trend is for steel manufacturers to attempt to manufacture a high strength steel to secure safety while reducing the weight of steel sheet used as a material of an automobile.
Recently, according to the current trend, the demand for a high strength hot dip galvanized steel sheet for an automobile is greatly increasing. In general, however, while a method of manufacturing high strength steel using a solid solution strengthening element such as P, Mn, or the like may go some way toward strengthening the steel and reducing the weight thereof, the method has a limitation in processing various forms of vehicle components.
Therefore, at the time of producing vehicle components, steel, which is able to be used for vehicle components having complicated forms through excellent processibility and to provide relatively high strength characteristics after the completion of the process, is required. As this sort of steel there is Advanced High Strength Steel (AHSS) such as dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), or the like. The AHSS may contain large quantities of elements such as Si, Mn, Al, and the like. Si is an element capable of maintaining ductility in steel without significantly reducing the strength thereof. Due to this reason, Si may be frequently used.
However, when Si as an alloying element of steel is added in an amount of approximately 0.1 wt % or more, a hot dip galvanized steel sheet manufactured through a general method may result in the occurrence of defects such as a bare spot or defects on an external part thereof occurring relatively easily. These defects are generated in an annealing process atmosphere as one of a plurality of processes for manufacturing the hot dip galvanized steel sheet. In the case of the annealing process, a heat treatment at a high temperature of approximately 800° C. is performed while maintaining a reduction atmosphere containing hydrogen of 5 vol % or more and nitrogen with regard to the remainder thereof (please refer to JP1999-323443 and U.S. Pat. No. 5,137,586). During the high temperature heat treatment process, Si may diffuse on the surface of the steel sheet. That is, the density of Si on the steel surface may be 10 to 100 times higher than an average density of Si in the entire steel sheet, and the surface of the steel sheet, enriched with Si, may react to moisture or impurities in an atmosphere of a furnace to form an SiO2 oxide film.
The SiO2 oxide film formed on the surface of the steel sheet in the process for manufacturing the hot dip galvanized steel sheet may seriously degrade a specific characteristic, the wettability of the steel sheet, such that it is therefore difficult to secure excellent wettability of the steel sheet, by which a bundle form of a bare spot effect occurs, or even when the plating process is properly undertaken on the surface thereof, the adhesion extent with regard to the plating state may become seriously degraded. That is, at the time of conversion processing the steel into components, the SiO2 oxide film may be a factor of a plating peeling phenomenon in which a plating layer is detached.
For reference, surface enrichment due to Si, Mn, or the like may occur by the following reaction formula.Fe2O3+3H2→2Fe+3H2OSi+2H2O→SiO2+2H2 Mn+H2O→MnO+H2  [Reaction Formula](Here, Fe2O3:FeO, Fe3O4, Fe(OH)x, O and other oxides)
In order to prevent defects from occurring due to the decrease in wettability of a high strength steel sheet due to Si, Mn, or the like, various techniques have been proposed, for example, there was provided the technique of increasing an amount of Al in the hot dip galvanizing bath to increase a production amount of a Zn—Fe—Al—Si based and Fe—Al—Si based alloying layer on an interface between Fe and an alloying layer. Since the alloying layer resolves an oxidized layer of an annoying element, a hot dip plating wettability decrease occurring due to the oxidized film of the alloying element on the interface can be suppressed. However, absolutely increasing the amount of Al within the plating bath may be undesirable, as the increase of Al may be a factor in intergranular corrosion, together with Pb inevitably added as an impurity to the plating bath at the time of manufacturing a mini-spangle steel sheet. The intergranular corrosion may cause the plating peeling, and moreover, since the increase of Al within the plating bath is not good for welding at the time of processing the steel sheet, the above-mentioned technique according to the related art actually has difficulties when practically applied.
In addition, according to another technique of the related art, in order to improve the wettability of Si-containing steel, there has been proposed a technique in which surplus air is introduced to a direct fired furnace to form an oxidized film, and then, a reduction process is performed in the heating furnace RTS of a 10 vol % H2-90 vol % H2 reduction atmosphere, to greatly increase wettability. As an example, when the thickness of an iron oxide may be increased by increasing the rate of air from the general rate of 0.9 to 1.05 in the direct fired furnace, and when a reduction heat treatment is performed therein, a pure iron layer is formed on the surface of a steel sheet; stabilized wettability can be secured. However, this technique according to the related art also has technical defects, that is, when the thickness of the oxidized film cannot be precisely controlled, plating peeling may occur due to the thickened film layer. To the contrary, since the oxidized film is thin and thus completely returned by the reduction process, Si is enriched intact on the surface of the steel sheet such that a zinc plating layer cannot be strongly adhered to the surface of the steel sheet or bare plating may occur thereon. Therefore, the thickness of the iron oxide should be precisely controlled in the direct fired furnace.
One of the above-mentioned techniques according to the related art regarding the oxidation-reduction heat treatment is disclosed in JP2001-226742. In this case, the oxidized film is formed with a thickness ranging from 0.02 μm to 1 μm during an oxidation heat process, and is then completely resolved during the reduction process, to secure the wettability thereof. In addition, in the cases of JP1994-172953 and JP1994-172954, an oxidized film retains a thickness ranging from 0.02 μm to 0.2 μm after the reduction heat treatment, but it is shown to be completely resolved by Al within the plating bath.
However, in the case of the above patent technologies with regard to the oxidation-reduction method, precisely controlling a composition of iron oxide formed at the time of oxidation heat treatment, and a thickness thereof, as well as a composition of iron oxidized film remaining after the reduction heat treatment, the porosity thereof, and the like is not easy. Therefore, a large difference in wettability is inevitable, according to working conditions or other external factors.
An aspect of the present invention provides a steel sheet annealing device for providing an advanced hot-dip plated steel sheet and a galvannealed steel sheet, a plated steel sheet manufacturing apparatus including the same, and a plated steel sheet manufacturing method using the same.